This invention relates to a directional drilling tool, and more particularly to a rotary-percussion device for directional drilling in rock.
A conventional drill for use in rock comprises a drill rod with a drilling head having a roller bit, which comprises three toothed conical steel elements with welded-on hard-metal (e.g. tungsten carbide) tips. The drill rod is hollow and during drilling a flushing liquid (referred to as xe2x80x9cdrilling mudxe2x80x9d) is pumped through the rod, exits around the roller bit and travels back through the drill hole. Attempts have been made to recycle drilling mud, but this requires complex filtration equipment, and disposal of drilling mud presents environmental problems.
Directional drilling in rock, in particular horizontal drilling, may employ a combination of rotary and percussion devices. The percussion device may be a pneumatically operated percussion hammer. Such devices may employ a wedge-shaped head. For straight-line drilling, the head is caused to rotate, and the rock is drilled by a combination of the rotary and percussive actions, together with a forward pushing action on the drilling tool. If rotation is stopped, the percussive and pushing actions cause the tool to describe a forward curve in view of its wedge-shaped head. Such tools commonly include an electronic transmitter (sonde) or the like, which cooperates with a receiver above ground for continuously determining the position and direction of the tool.
A common procedure is to first drill a pilot bore. This bore may then be widened to accommodate pipes. cables etc. which are to pass through it. Widening may be carried out by xe2x80x9cbackreamingxe2x80x9d, i.e. passing a tool (xe2x80x9creamerxe2x80x9d) backwards through the pilot bore. A compacting reamer is a wedge- or cone-shaped tool which can be pulled back through the pilot bore, optionally with rotation. A xe2x80x9cflyxe2x80x9d reamer is a tube or rod with external blades, which is pulled back through the pilot bore, also with rotation. Compacting reamers are generally unable to work in rock, and fly reamers can be slow to cut away at hard rock. Attaching a percussive hammer with compacting reamers to the drill rod for backreaming is known, but is only effective in soft ground.
An object of the present invention is to provide a complete drilling tool which is shaped and constructed in such a way as to provide a particularly effective directional drilling and backreaming action in rock or similar hard media.
The present invention provides a rotary-percussion device for directional drilling and backreaming in rock. For pilot bore drilling it comprises a substantially cylindrical, rotatable body portion housing a percussion hammer, and a frustoconical head eccentrically mounted on the front of the body portion, wherein the minimum diameter face of the head is adjacent to the body portion, the axis of the head is parallel to but offset from the axis of the body, and the front face of the head is chisel-shaped and comprises at least one oblique plane sloping forwardly in the same direction as the offset of the head axis relative to the body axis.
An airline preferably passes through the body and optionally through the head, exiting on the front face of the body or the head in the forwardmost half thereof. In use of the device, pressurised air passes through this airline, exits at the front face and passes back through the drill hole, carrying with it broken fragments of rock and soil (cuttings). The use of drilling mud is accordingly avoided.
The head may be integral with a neck portion which is slidingly received in a forwardmost part of the body. A plurality of hard studs (for example of tungsten carbide) is preferably arranged on the front face of the head, at least in the forwardmost half thereof.
Longitudinally extending, peripheral slots or grooves may be present in the head, to form an interrupted cutting face and to allow the passage of air or cuttings.
The front face of the head may comprise a single flat surface which forms an oblique plane, the forwardmost side of which is offset from the centre in the same direction as the offset from the body axis to the head axis. Alternatively, the front face of the head may comprise a non-oblique or only slightly oblique, forwardmost portion on the offset side, and a more oblique portion on the side remote from the offset.